The present invention is directed to a power transistor having a plurality of active transistor regions and is referred to as a multifinger transistor.
Power transistors are usually composed of individual transistors that multiply integrated side-by-side and that are provided with finger-shaped contacts. These power transistors are therefore usually referred to as multifinger transistors. An output power that is higher in comparison to individual transistors is achieved on the basis of this joining. Such a finger-shaped contact can lead to the gate of a field effect transistor (particularly a HEMT) or to an emitter of a bipolar transistor.
During operation, power transistors emit dissipated power as heat. In the case of multifinger transistors, this leads to an overheating of the middle transistors of a row of individual transistors as compared to the transistors that are located at the outside. The non-uniform heating of the individual transistors in multifinger transistors was hitherto mensurationally acquired, analyzed and described by theoretical models. A non-uniform heating can be compensated by electrical circuit measures such as, for example, resistors in the emitter circuit (see, for example, G.-B. Gao et al., "Thermal Design Studies of High Power Heterojunction Bipolar Transistors.revreaction., IEEE Trans. ED-36 (5) 1989, pages 854 and G.-B. Gao et al. "Emitter Ballasting Resistor Design for, and Current Handling Capability of AlGaAs/GaAs Power Heterojunction Bipolar Transistors", IEEE Trans. ED-38 (2), 1991, page 185). The allowable maximum temperature for the individual transistors lying in the middle of a row limits the maximum power input of the multifinger transistor.
In the design of integrated circuits an attempt is made to place temperature-sensitive circuit parts on isotherms (see, for example, G.-B. Gao et al., "Uniform Junction Temperature of AlGaAs/GaAs Power Heterojunction Bipolar Transistors on Silicon Substrate", Appl. Phys. Lett. 58 (10) March 1992, page 1068).
The heating of the individual transistors becomes increasingly non-uniform, the lower the thermal conductivity of the appertaining semiconductor material. As a consequence of the lower thermal conductivity of gallium arsenide (GaAs) compared to silicon, a less uniform heating than in the case of silicon can be anticipated given employment of GaAs. The various coefficients of thermal conductivity for semiconductor materials may be derived from the applicable tables.
Active transistor regions, particularly those of power transistors, represent heat sources that are for all practical purposes only two-dimensionally present at a surface of the semiconductor body. R.D. Lindsted, R.J. Surty, "Steady State Junction Temperatures of Semiconductor Chips:, IEEE Trans. ED-19(1), 1972, page 41, describes a calculation for a threedimensional, static temperature simulation from which the temperature distribution is derived on the semiconductor surface on which an individual transistor is present. The third spatial dimension lying perpendicularly thereto is required for describing the heat elimination by cooling members.